Method of controlling a plurality of compressors

ABSTRACT

Compressed gases are gathered from a plurality of compressors and employed if required. A pressure of compressed gas thus gathered is measured by a pressure sensor to generate a pressure signal, which is transmitted to a control. The signal is transmitted from the control to electromagnetic switches to control motors for the compressors to determine which of the compressors run.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of controlling aplurality of compressors.

[0002] Japanese Patent No. 2,875,702 discloses a method of controlling aplurality of compressors in a system in which compressed gases from aplurality of compressors are gathered into a pressure tank of which arequired compressed gas is taken out.

[0003] When compressed gases outputted from compressors are suppliedinto a pressure tank of which the compressed gas is taken out in use,the compressors frequently start and stop, which results in failure inthe compressors and burning damage in a motor if difference in pressurebetween the upper and lower limits is small.

[0004] Accordingly, with increase in the volume of the pressure tank,the upper limit of pressure is kept to 0.2 MPa higher than used pressureto increase difference between the upper and lower limits.

[0005] However, in this method, it operates for a long time to increasedriving power thereby increasing running costs. The compressors arecontinuously operated at a high pressure to cause decrease in lives ofthe compressors.

[0006] Furthermore, rotation speed of the compressor is set depending onhigh upper limit pressure and it is necessary to decrease rotation speedduring normal operation. So it operates with decreased amount of air,thereby decreasing efficiency of the compressors.

[0007] The compressors are cooled with cooling fans driven at lowrotation speed thereby decreasing cooling effect. Thus, it causesdecrease in lives of tip seals and bearings.

SUMMARY OF THE INVENTION

[0008] In view of the above disadvantages, it is an object of theinvention to provide a method of controlling operation of compressors toincrease the number of the compressors operated at low pressure closerto the lower limit and to decrease the number of the compressorsoperated at higher temperature closer to the upper limit, time foroperation being decreased to decrease running costs to improve the livesof the compressors, tip seals and bearings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above and other features and advantages of the invention willbecome more apparent from the following description with respect toembodiment as shown in appended drawings wherein:

[0010]FIG. 1 is a view which shows one embodiment of the presentinvention; and

[0011]FIG. 2 is a graph which shows a change in pressure to compressorsin FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0012]FIG. 1 illustrates one embodiment of the present invention.

[0013] Compressed gases, such as compressed air, outputted from aplurality of compressors are gathered into a pressure tank. A requiredamount of compressed air is taken out of the pressure tank and employed.The compressed gases are not always gathered into the pressure tank, butmay be taken out of conduits directly.

[0014] In the embodiment as shown in FIG. 1, eight compressors 1comprising C₁ to C₈ are driven by motors 2 comprising M₁ to M₈respectively, Electromagnetic switches 3 comprising E₁ to E₈ turn themotors M₁ to M₈ on and off respectively.

[0015] At exits of the compressors 1, check valves 4 comprising V₁ to V₈are provided to prevent running back of compressed air from thecompressors 1 Compressed air through the check valves 4 is all fed intoa pressure tank 5, of which compressed air used in an object 7 is takenout by opening and shutting a valve 6. To keep compressed air in thepressure tank 5 at a desired pressure, the compressors 1 are controlledon the basis of feedback by determination of compressed air in thepressure tank 5.

[0016] Pressure of compressed air in the pressure tank 5 is measured bya pressure sensor 8 to generate a pressure signal which is transmittedto a control 9 which has a determining portion 10 for determining thenumber of operating compressors and a selecting portion 11 for startingand stopping any of the compressors. Based on the pressure signalmeasured by the pressure sensor 8, the determining portion 10 determinesthe number of the operating compressors 1. Generally, the operatingcompressors are determined depending on the pressure signal betweenpredetermined upper and lower limits.

[0017] In a preferred embodiment, each of the compressors 1 hasapproximately the same capability, and the pressure between the upperand lower limits are divided into pressure levels which are equal to thecompressors 1 in number. The measured pressure signal is out of desiredpressure level and reaches to the boundary of the pressure levels,thereby changing the number of the operating compressors 1.

[0018] Specifically, the measured pressure signal increases and reachesto pressure level higher than desired pressure level by one stage. Then,the number which is subtracted by one from the number of the operatingcompressors 1 is determined as a new number of operating compressors.Meanwhile, the pressure signal decreases and reaches to pressure levellower than desired pressure level by one stage. Then, the number whichis added by one to the number of the operating compressors 1 isdetermined as a new number of operating compressors.

[0019] A signal of the number determined in the determining portion 10is transmitted to the selecting portion 11 that selects whichcompressors start or stop. The selection is carried out withpredetermined order of starting and stopping. Preferably, numeric ordermay be allotted to the compressors 1 which start and stop in the order.By the numeric order, any of the compressors 1 equally start and stop toavoid disadvantage that a specific compressor frequently starts andstops or is operating for a long time.

[0020] When the compressor 1 which starts or stops is selected in theselecting portion 11, a signal is transmitted to the operating portion12 from which it is transmitted to the electromagnetic switches 3 whichturn on or off power sources to the corresponding motors 2, so that thecompressors 1 selected in the selecting portion 11 start or stop.

[0021]FIG. 2 is a graph that illustrates changes in pressure to theoperating compressors 1 when the eight compressors 1 such as C₁ to C₈are controlled. Compressed air in the pressure tank 5 between the lowerlimit such as 0.6 MPa and the upper limit such as 0.7 MPa is divided inpressure into eight stages which are equal to the compressors 1 innumber to determine each pressure level. The lower and upper limits areset to level 0 and level 8/8 respectively, and 1/8 to 8/8 are settherebetween by dividing equally. Also, for example, desired pressurelevels are set to 1/8 to 2/8. Continuous Nos. 1 to 8 are allotted to theeight compressors 1.

[0022] As shown in FIG. 2, first, until a pressure signal 13 measured bythe pressure sensor 8 becomes the pressure level 1/8, the eightcompressors 1 all run. Pressure rises, the pressure level reaches 1/8 atP1 beyond the lower limit 0, and a stop signal is transmitted from thecontrol 9 to the electromagnetic switch E₁ of the compressor No.1 (C₁)via the operating portion 12 to stop the compressor No.1. The sevencompressors Nos. 2 to 8 still run.

[0023] Pressure rises without compressed air to reach the pressure level2/8 at P2, and then a stop signal is transmitted to the electromagneticswitch E₂ for the compressor No.2 (C₂) from the control 9 via theoperating portion 12 to stop the compressor No.2. The six compressorsNos. 3 to 8 still run.

[0024] Pressure rises slightly without compressed air. Before it becomesthe next pressure level 3/8, compressed air is used and the pressurelevel falls to 1/8 at P3, and a start signal is transmitted from thecontrol 9 to the electromagnetic switch E₁ for the compressor No.1 (C₁)which initially stops, via the operating portion 12, so that thecompressor No.1 starts. Thus, the seven compressors No.1 and Nos. 3 to 8still run.

[0025] With compressed air, the pressure level falls, but does not reach0. Thereafter, compressed air is not used, and pressure reaches to 2/8at P4 again beyond the pressure level 1/8. A stop signal is transmittedto the electromagnetic switch E₃ for the compressor No.3 (C₃) from thecontrol 9 via the operating portion 12. Thus, the six compressors No.1and Nos.4 to 8 still run.

[0026] Compressed air is not used and the pressure level rises to 3/8 atP5. A stop signal is transmitted from the control 9 to theelectromagnetic switch E₄ for the compressor No.4 (C₄) via the operatingportion 12 to stop the compressor No.4. The five compressors No.1 andNos. 5 to 8 still run.

[0027] Compressed air is not used and pressure rises, so that thepressure level reaches to 4/8 at P6. A stop signal is transmitted fromthe control 9 to the electromagnetic switch E₅ for the compressor No.5(C₅) via the operating portion 12 to stop the compressor No.5. The fourcompressors No.1 and Nos. 6 to 8 still run.

[0028] Compressed air is not used and pressure rises, so that thepressure level reaches to 5/8 at P7. A stop signal is transmitted fromthe control 9 to the electromagnetic switch E₆ for the compressor No.6(C₆), which stops. The three compressors Nos. 1, 7 and 8 still run.

[0029] Compressed air is not used, and pressure rises, so that thepressure level reaches to 6/8 at P8. A stop signal is transmitted fromthe control 9 to the electromagnetic switch E₇ for the compressor No.7(C₇) via the operating portion 12 to stop the compressor No.7. The twocompressors Nos. 1 and 8 still run.

[0030] Compressed air is not used and the pressure level rises, so thatthe pressure level reaches to 7/9 at P9. A stop signal is transmittedfrom the control 9 to the electromagnetic switch E₈ for the compressorNo.8 (C₈) via the operating portion 12 to stop the compressor No.8. Onlythe compressor No.1 still runs.

[0031] Compressed air is not used and pressure rises, so that thepressure level reaches to 8/8 at P10. A stop signal is transmitted fromthe control 9 to the electromagnetic switch E₁ for the compressor No.1(C₁), which stops. All the compressors do not run.

[0032] While compressed air is used and the pressure level falls below7/8, all the compressors do not run yet.

[0033] Compressed air is used and the pressure level falls to 7/8 atP11. A start signal is transmitted from the control 9 to theelectromagnetic switch E₂ for the compressor No.2 (C₂) which secondlystops, via the operating portion 12, to start the compressor No.2. Theseven compressors No. 1 and Nos. 2 to 8 do not run yet.

[0034] Compressed air is used, and the pressure level falls to 6/8 atP12. A start signal is transmitted from the control 9 to theelectromagnetic switch E₃ for the compressor No.3 (C₃) which thirdlystops, via the operating portion 12, to start the compressor No.3. Thecompressors No. 1 and Nos.4 to 8 do not run yet.

[0035] Compressed air is used and the pressure level falls to 5/8 atP13. A start signal is transmitted from the control 9 to theelectromagnetic switch E₄ for the compressor No.4 (C₄) which stopsfourthly, via the operating portion 12 to start the compressor No.4. Thefive compressors No.1 and Nos. 5 to 8 do not run.

[0036] Compressed air is used and the pressure level falls to 4/8 atP14. A start signal is transmitted from the control 9 to theelectromagnetic switch E₅ for the compressor No.5. (C₅) which stopsfifthly, via the operating portion 12 to start the compressor No.5. Thecompressors No.1 and Nos.6 to 8 do not run yet.

[0037] Compressed air is used and the pressure level falls to 3/8 atP15. A start signal is transmitted from the control 9 to theelectromagnetic switch E₆ for the compressor No. 6 (C₆) which stopssixthly, via the operating portion 12 to start the compressor No.6. Thethree compressors Nos. 1, 7 and 8 do not run.

[0038] Compressed air is used and the pressure level fall to 2/8 at P16.A start signal is transmitted from the control 9 to the electromagneticswitch E₇ for the compressor No.7 (C₇) which stops seventhly, via theoperating portion 12 to start the compressor No.7. The two compressorsNos. 1 and 8 do not run yet.

[0039] Compressed air is used and the pressure level fall to 1/8 at P17.A start signal is transmitted from the control 9 to the electromagneticswitch E₈ for the compressor No.8 (C₈) which stops eighthly, via theoperating portion to start the compressor No.8. Only the. compressor No.1 does not run yet.

[0040] Compressed air is used and the pressure level falls to 0/8 atP18. A start signal is transmitted from the control 9 to electromagneticswitch E₁ for the compressor No.1 (C₁) which stops ninthly, via theoperating portion 12 to start the compressor No.1. All the compressorsNos. 1 to 8 run, and there is no compressor that does not run.

[0041] The foregoing merely relate to embodiments of the invention.Various changes and modifications may be made by a person skilled in theart without departing from the scope of claims wherein:

What is claimed is:
 1. A method of controlling a plurality ofcompressors, comprising the steps of: gathering compressed gasesoutputted from said plurality of compressors which have approximatelythe same capability; measuring pressure of a compressed gas thusgathered to transmit a pressure signal to control said plurality ofcompressors; dividing the pressure of the compressed gas between upperand lower limits into a plurality of pressure levels which are equal innumber to said compressors; and determining which of the compressors runwhen the pressure signal reaches the boundary of said pressure levels.2. A method as claimed in claim 1 wherein the compressed gases aregathered into a pressure tank from said plurality of compressors.
 3. Amethod as claimed in claim 1 wherein the pressure of the gatheredcompressed gas is measured by a pressure sensor to generate a pressuresignal.
 4. A method as claimed in claim 1 wherein the compressed gas isair.
 5. A method as claimed in claim 1 wherein the pressure signal istransmitted to a control which has a determining portion for determiningthe number of operating compressors and a selecting portion for startingand stopping any of the compressors.
 6. A method as claimed in claim 5wherein the pressure signal is transmitted from the control toelectromagnetic switches via an operating portion, said electromagneticswitches being connected to motors for said compressors to turn on andoff said compressors.
 7. A method as claimed in claim 1 wherein thepressure rises to an upper pressure level to decrease the number ofoperating compressors, while the pressure falls to a lower pressurelevel to increase the number of operating compressors.
 8. A method asclaimed in claim 1 wherein continuous numbers are allotted to saidplurality of compressors, any of the compressors being started andstopped in order of the numbers to change the number of operatingcompressors.